Ancient evils, madness causing knowledge and sights, ontologically distorted things and events that will not conform to the mundane reality everyone expects. Stuff that rips your expectations of reality out by the roots and dangles their twitching guts in front of your exploded eyeballs so your shredded soul can see it. That kind of thing.
In all cases a Bad Thing and best avoided on the worse of days. Big Sur Mental Health Clinic and like establishments are populated by people that have been there and been done by that. Do avoid becoming one of them.